Electrical bushing having adjacent capacitor sections separated by axially continuous conductive layers, and including a cooling duct

ABSTRACT

A condenser bushing including a conductor, electrical insulation disposed about the conductor, and a plurality of radially spaced cylindrical layers of electrically conductive material disposed in the electrical insulation. The cylindrical layers of electrically conductive material are arranged to provide at least first and second concentric radially adjacent condenser systems, with at least one of the systems being of the split or axially divided type. The first and second condenser systems are separated by a continuous electrically conductive layer, not structurally related to either adjacent system, but which functions as the outermost layer of the first condenser system, and the innermost layer of the second condenser system, to connect the two systems in series.

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IOR ART ELECTRICAL BUSHING HAVING ADJACENT CAPACITOR SECTIONS SEPARATEDBY AXIALLY CONTINUOUS CONDUCTIVE LAYERS, AND INCLUDING A COOLING DUCTBACKGROUND OF THE INVENTION 1. Field of the Invention The inventionrelates in general to electrical bushings, and more specifically tocondenser bushings of the type suitable for use with power transformersand power circuit breakers.

2. Description of the Prior Art Condenser type bushings of the priorart, such as those used with power transformers and power circuitbreakers, have a plurality of radially spaced cylindrical layers ofelectrically conductive material disposed in electrical insulation aboutthe bushing conductor, to grade radial and longitudinal voltagedistribution throughout the insulating material of the bushing.

Designs for the condenser section of EHV (extra high voltage) bushings,when using the teachings of the prior art, are often impractical orextremely large in diameter. This is due to the large external andinternal clearances required between the top and bottom terminal ends ofthe bushing and an intermediate metallic flange assembly which is usedto mount the bushing to a grounded metallic casing of associatedelectrical apparatus. The length of the grounded metallic flangeassembly is kept as short as possible, so that it does not addunnecessarily to the overall length of the bushing, as bushing length isalready a problem in EHV bushings from the standpoint of mounting thebushings and supporting them. Further, the length of the outermostconductive layer of the condenser section, which is grounded duringnormal operation of the bushing, is short compared with the length ofthe innermost conductive layer. This high ratio of the innermost to theoutermost conductive layer length results in capacitor systems which donot make the most economical use of the bushing insulation, and which donot provide the required control of voltage gradients in the bushing.

Increasing the length of the outermost or ground layer of the capacitorsection, or shortening the extension of the capacitor section into theupper porcelain housing, to provide a more favorable ratio of the lengthof the inner layer to the length of the outer layer, is not a solutionto the problem as the distribution of the voltage across the length ofthe external porcelain is adversely affected, increasing the probabilityof flashover.

The use of split or axially divided capacitor layers between the bushingconductor and the ground layer, such as disclosed in US. Pat. Nos.3,394,455 and 2,462,545, both of which are assigned to the same assigneeas the present application, has been beneficial in EI-IV bushings, as itreduces the required radial build of the condenser section. However, theproblems hereinbefore set forth are encountered, even with the splitcapacitor arrangement, at the upper end of the present EHV voltagerange.

SUMMARY OF THE INVENTION Briefly, the present invention is a new andimproved condenser bushing which more uniformly stresses the insulationof the capacitor section and provides the desired control of voltagegradients within the bushing, without increasing the length of theground layer, and without reducing the extension of the capacitorsection into the upper porcelain housing. This is accomplished byutilizing a plurality of separate concentric, radially adjacentcondenser systems, with at least one of the systems being of the axiallydivided or discontinuous type. At least one continuous, electricallyconductive layer is disposed between adjacent condenser systems, withthe continuous layer being independent from the specific structuralcapacitor arrangements of the adjacent systems, thus enabling it tofunction as the outermost layer of the adjacent inner capacitor system,and the innermost layer of the adjacent outer split capacitor system.This provides a structure in which the capacitor system has twocapacitors effectively connected in parallel, and the parallel connectedcapacitors are serially connected with the capacitor, or capacitors ofthe other radial systems, between the bushing conductor and the groundlayer of the capacitor system. The radially disposed capacitor systemsare constructedto each control a desired percentage of the total voltagestress, enabling much closer control over the voltage gradients withinthe bushing, and thus more effectively utilizing the electricalinsulation of the bushing.

The disclosed construction also facilitates disposing cooling ductsthrough the condenser section, as the cooling ducts may be disposedimmediately adjacent the continuous electrically conductive layer whichseparates the different capacitor systems, and another continuous layerof electrically conductive material may be disposed on the other side ofthe duct. Interconnection of the two continuous layers reduces themagnitude of electrical stress in the duct, preventing breakdown of thecooling oil which flows therethrough.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be betterunderstood, and further advantages and uses thereof more readilyapparent, when considered in view of the following detailed descriptionof exemplary embodiments, taken with the accompanying drawings, inwhich:

FIG. 1 is an elevational view, partially in section, of a condenserbushing of the type which may utilize the teachings of the invention;

FIG. 2 is a schematic representation of a prior art condenser bushingarrangement;

FIG. 3 is a partial diagrammatic representation of a condenser sectionconstructed according to the teachings of the invention, which may beused with the condenser bushing shown in FIG. 1; and

FIG. 4 is a schematic representation of the condenser section shown in FIG. 3.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, andFIG. 1 in particular, there is shown an elevational view of a condenserbushing 10 of a type which may utilize the teachings of the invention.Bushing 10, which may be used with power transformers or power circuitbreakers, has a generally elongated shape, and includes a centrally oraxially disposed electrical conductor 12, which is constructed of a goodelectrical conductor, such as copper or aluminum. Conductor 12 isterminated at its upper end by an expansion bowl 13 and a suitableterminal cap assembly 14, which is adapted for connection to an externalelectrical conductor, and at its bottom end by terminal means 16, whichis adapted for connection to a conductor of the associated electricalapparatus. The electrical conductor 12 has an insulating or condensersection 18 disposed thereon, which may be formed of layers of insulatingmaterial having a plurality of spaced metallic foil sheets 20 disposedtherein to form cylindrical capacitor plates to grade the radial andlongitudinal voltage distribution in the bushing. The insulating orcondenser section 18 may be formed by winding a high quality paper onthe electrical conductor 12 while the paper is under a predetermineduniform tension, and feeding in metallic foil sheets or capacitor platesat predetermined intervals to provide a plurality of serially connectedcapacitors. Since it is desirable to unifonnly grade the radial voltageacross condenser section 18, the capacitance of each section may beconstructed to be equal. For example, as the diameter of the capacitorplates 20 increases, the longitudinal length of the plates may decreasein order to maintain substantially the same surface area. The graduatedlength of the foil layers 20 provides graded voltage longitudinally oraxially along the length of the bushing.

The condenser section 18 is oil impregnated to fill all of the voids inthe structure and prevent the formation of corona discharges therein, toprovide a minimum power factor and thus reduce dielectric heating of thestructure, and in certain ratings it includes one or more ducts forcirculation of the oil to remove heat from the condenser section.

A grounded metallic flange assembly 22 is disposed intermediate the endsof the bushing and adjacent to the condenser section 18, which includesa metallic cylindrical flange tube member 23 and connected flange member25, providing means for attaching the bushing 10 to a metallictransformer or circuit breaker casing 27. Shell-like insulating members24 and 26, which may be constructed of an electrical grade of porcelainhaving a glazed, corrugated outer surface for providing additional creepdistance, or of a suitable resin system, such as an epoxy resin, aredisposed between the flange assembly 22 and the upper terminal 14, andbetween the flange assembly 22 and the bottom terminal 16, respectively.The ends of the shell members 24 and 26 are machined flat and parallelto provide surfaces for sealing gaskets, such as gaskets 28, 29, 30 and31, for hermetically sealing the bushing 10. Shell members 24 and 26provide weatherproof insulating means between the ends of the bushingand central grounded flange assembly 22, and also provide a containerfor the insulating and cooling fluid 32, such as mineral oil, with whichthe bushing 10 is filled.

While bushing 10 has been described as having a capacitor section formedof layers of paper interspersed with radially spaced layers ofelectrically conductive material to provide capacitor plates, it is tobe understood that the condenser section 18 may be constructed in anyother suitable manner, such as by winding the capacitor section withresin impregnated paper, such as epoxy resin, adding sheets of foil asrequired, or by placing the capacitor plates in a mold and introducing aliquid resinous insulation system thereto, such as an epoxy or polyesterresin, which resin system is subsequently cured to a solid. In thelatter instance, the entire insulating structure of the bushing,including the weather sheds, may be formed of cast resin. Further, whilethe capacitor plates are broadly termed electrically conductive, it isto be understood that they may be formed of sheets, foils, or coatings,of a good electrical conductor, such as copper or aluminum, they may besheets or coatings of partially conductive material, such as thosecontaining carbon, or they may be sheets or coatings of semiconductivematerial, i.e., material having a distinct voltage dependentresistivity, such as those containing silicon carbide.

The extra high voltages (EI-IV) i.e., above 230 KV, now being generatedby electrical utilities, require that the apparatus associatedtherewith, such as power transformers and power circuit breakers, haveunusually long electrical bushings. This is due to the dimensions of theclearances required between the terminals 14 and 16 and the groundedflange assembly 22. Thus, the length of the flange tube 23 must be keptas short as possible so it does not add unnecessarily to the bushinglength. However, keeping the flange 23 as short as possible restrictsthe length of the outermost conductive layer of the condenser section,which is the ground layer of the section. The length of the innermostconductive layer is dictated by the length of the bushing conductor, andhence is very long. This high ratio of the length of the innermost tothe outermost conductive layers results in poor utilization of thebushing insulation, highly stressing some areas of the insulation whileother areas are subject to very little stress, as the control of voltagegradients within the bushing is less than optimum. Reducing theextension of the condenser section 18 into the upper portion of housing24, or increasing the length of the outermost conductive layer of thecapacitor section 18, in order to obtain a more favorable ratio,adversely affects the distribution of the voltage across the externalsurfaces of the porcelain housings, increasing the chances of flashover.

The split or axially divided condenser section, such as shown in thehereinbefore mentioned United States patents, has been an excellentarrangement for extra high voltage bushings, resulting in two capacitorseffectively connected in parallel between the bushing conductor andgrounded flange assembly. FIG. 2 schematically illustrates a prior artbushing assembly 39 having a split condenser section represented bycapacitors 40 and 42 connected in parallel between the bushing conductor44 and the grounded flange assembly, which is given the referencenumeral 46.

While the prior an bushing arrangement shown in FIG. 2 has beensuccessfully employed in HIV bushings, when the bushings are designedfor the upper range of EHV voltages, even the split condenserarrangement results in less than optimum utilization of the bushinginsulation.

FIG. 3 is a diagrammatic representation of a condenser section 50, whichmay be used in bushing 10 shown in FIG. 1, or any other suitable bushingconstruction, with the condenser section 50 being constructed accordingto the teachings of the invention. Condenser section 50 obtains a moreefficient usage of the bushing insulation, and a more linear voltagedistribution across the porcelain housings 24 and 26, without increasingthe length and diameter of the bushing.

More specifically, it has been found that by using two or moresuperposed condenser sections, at least one of which is of the split oraxially divided type, with adjacent sections separated by an axiallycontinuous layer of conductive material, that excellent control isprovided over voltage gradients in the bushing, resulting in moreuniformly stressing the bushing insulation, and more linearlydistributing voltages across the porcelain housings, without increasingthe length of the outermost or grounded layer of the condenser section,or reducing the length of the innermost layer. The plurality ofindividual condenser sections may be formed progressively on the bushingconductor, or they may be constructed separately and telescoped into thedesired positions. Each axial half of any split type condenser sectionmay be formed individually, if desired, and telescoped into position,with each cylindrical layer in each half being aligned with acylindrical layer in the other half, or offset, as desired.

FIG. 3 diagrammatically illustrates a condenser section 50 having first,second and third radially superposed condenser systems 52, 54 and 56,respectively, all of the axially divided type but it is to be understoodthe invention applies to two or more condenser systems, at least one ofwhich is of the divided type, with the total number of condensersections used being determined by the BIL voltage rating of the bushing.Radially adjacent condenser systems are separated by a continuous layerof conductive material, which serially interconnects the capacitors ofthe adjacent condenser systems. The axially continuous separating layersare not structurally part of either adjacent system, but may be formedof the same material of which the plates of the adjacent condensersystems are formed.

The radially adjacent condenser systems are illustrated in FIG. 3 asbeing separated by two spaced continuous layers, because the new andimproved condenser system 50 facilitates the placement of substantiallyvertical cooling ducts, i.e., parallel with the longitudinal axis of thebushing through the condenser system, which ducts are required incertain bushing ratings, with the continuous layers being placedadjacent to the inner and outer sides of the ducts, and interconnectedto provide substantially the same voltage on each side of the duct.

More specifically, condenser assembly or section 50, which issymmetrical about centerline 58, is disposed about a central or axiallyextending conductor 60, with an oil duct 62 provided between theconductor 60 and condenser assembly 50, if required, which is shieldedby an axially continuous layer 64 which is connected to conductor 60 viaconnector 66.

The first condenser system 52 is designed to control a predeterminedpercentage of the total voltage, and for purposes of example, threeaxially divided layers of conductive material are illustrated, but inpractice many more layers will usually be utilized. The two axiallydivided cylindrical capacitor foils or plates of the first layer arereferenced 68 and 68', the two divided conductive layers of the secondlayer are referenced 70 and 70' and the axially divided conductors ofthe third layer are referenced 72 and 72'.

Condenser system 52, and the next radially adjacent system 54, areseparated by at least one continuous layer of electrically conductivematerial, which functions as the outermost layer of condenser system 52and as the innermost layer of condenser system 54. For purposes ofexample, a cooling duct 74 is shown disposed between the two condensersystems, with continuous layers 76 and 78 of conductive materialdisposed adjacent to the inner and outer surfaces of the cooling duct.The two continuous layers 76 and 78 are interconnected electrically, asshown at 79, to reduce electrical stresses across the duct, and theyalso function electrically as one continuous layer between the tworadially adjacent condenser systems.

The second condenser system 54 is designed to control a predeterminedpercentage of the total voltage to ground, and it is illustrated ashaving first, second and third axially divided layers referenced 80 and80', 82 and 82', and 84 and 84, respectively.

The second and third condenser systems 54 and 56 are separated by atleast one continuous layer of electrically conductive material, and asillustrated, a cooling duct 86 is provided between these two condensersystems, and hence two continuous conductive layers 88 and 90 areprovided, adjacent to the inner and outer sides of the duct 86 to shieldthe oil therein against excessive electrical stresses. The two layers 88and 90 are interconnected electrically, as shown at 92, to provide thesame voltage on each side of the duct, and the layers 88 and 90 thusfunction as a single layer, i.e., as the outermost layer of condensersystem 54, and as the innermost layer of condenser system 56.

The third and final condenser system 56, of this example is designed tocontrol a specific percentage of the voltage, and it includes threedivided layers referenced 94 and 94, 96 and 96' and 98 and 98',respectively, and an outer continuous layer 100, which functions as theground layer. Grounded layer 100 is connected to the metallic flangeassembly, which in turn is connected to the grounded metallic casing ofthe associated electrical apparatus, referenced 102.

The new and improved bushing condenser assembly 50 is illustratedschematically in FIG. 4, with the first condenser system 52 beingillustrated by capacitors 104 and 106 connected in parallel between thebushing conductor 60 and the continuous layers 76 and 78. The secondcondenser system 54 is illustrated schematically by capacitors 108 and110 connected in parallel between continuous layers 76, 78 andcontinuous layers 88, 90 and thus the parallel connected capacitors 108and 110 are serially connected with the parallel connected capacitors104 and 106.

The third condenser system 56 is illustrated schematically by capacitors112 and 114, connected in parallel between continuous layers 88, 90, andthe ground layer 100, and thus the parallel connected capacitors 112 and114 are serially connected with the parallel connected capacitors of thefirst and second condenser systems 52 and 54, respectively. Thisseries-parallel arrangement of capacitors across the bushing insulation,between the bushing conductor 60 and the ground layer 100, providesexcellent control of the voltage gradients within the bushing, moreuniformly stressing the bushing insulation, and more linearly gradingvoltage stress both radially and axially in the bushing. Further, thenew and improved bushing assembly provides these results without addingto the bushing length, or increasing the bushing diameter. An addedbenefit of the disclosed structure is the fact that cooling ducts may beprovided through the solid insulation of the conductor assembly withoutupsetting the capacitive structure, and without danger of exceeding theelectrical breakdown strength of the oil flowing through the ducts, asthe ducts may be placed between the plurality of radially disposedcondenser systems, and shielded by the continuous layers of electricallyconductive material disposed between the systems, which layers functionas the outer layer of the immediately adjacent inner system, and theinner layer of the immediately adjacent outer capacitor system.

While a preferred embodiment of the invention utilizes two or moreradially adjacent, axially divided type condenser systems, separated byone or more continuous layers, in certain applications an axiallydivided condenser system may advantageously be used with a system havingcontinuous layers, with a continuous layer disposed between the tworadially adjacent systems. For example, when there is no air space abovethe oil in the tank of the associated apparatus the ground layer is veryshort. Thus, in order to increase the capacitance of the condensersystem which includes the ground layer, it may utilize interleavedcontinuous layers, such as disclosed in copending application Ser. No.880,228, filed Nov. 26, 1969, now U.S. Pat. No. 3,600,502, which isassigned to the same assignee as the present application, with the nextadjacent system being of the split type, and the two systems separatedby continuous layers which are not part of the basic structuralarrangement of either adjacent condenser system.

Bushings were constructed according to the teachings of the invention,and successfully tested, for a transformer to be operated at a voltageof 1,150 KV, with the bushing rating being 2,175 KV BIL, using threeradially disposed split-type condenser systems, separated by axiallycontinuous layers. These bushings were constructed such that theinnermost capacitor system controlled 26.2 percent of the voltage, theintermediate capacitor system 32.4 percent, and the outer capacitorsystem 41.4 percent, but these values may be altered by changing thecapacitance in any of the individual capacitor systems.

I claim as my invention:

1. An electrical condenser bushing, comprising an axially extendingelectrical conductor,

a plurality of concentric, radially adjacent condenser systems disposedabout said electrical conductor,

at least one axially continuous layer of electrically conductivematerial disposed between at least two radially adjacent condensersystems, which layer is structurally independent from either adjacentsystem, and which functions to serially interconnect the adjacentcondenser systems,

and at least one cooling duct disposed between two adjacent condensersystems, with the at least one axially continuous layer disposedadjacent to one side of the duct, and including another axiallycontinuous layer disposed adjacent to the other side of said duct.

2. An electrical condenser bushing, comprising:

an axially extending electrical conductor,

electrical insulating means disposed about said conductor,

a plurality of radially spaced, axially discontinuous layers ofelectrically conductive material disposed in said insulating means,about said conductor,

a plurality of axially continuous layers of electrically conductivematerial, including a continuous layer disposed about the outermostaxially discontinuous layer, and at least one continuous layer disposedintermediate the axially discontinuous layers, to divide the axiallydiscontinuous layers into a plurality of radially disposed groups,

and at least one cooling duct disposed in the insulating means, withaxially continuous layers disposed immediately adjacent to the inner andouter sides of the at least one cooling duct.

3. An electrical condenser bushing, comprising:

an axially extending electrical conductor,

electrical insulating means disposed about said conductor,

a plurality of radially spaced, axially discontinuous layers ofelectrically conductive material disposed in said insulating means,about said conductor,

a plurality of axially continuous layers of electrically conductivematerial, including a continuous layer disposed about the outermostaxially discontinuous layer, and at least one continuous layer disposedintermediate the axi' ally discontinuous layers, to divide the axiallydiscontinuous layers into a plurality of radially disposed groups, acooling duct disposed adjacent to the conductor, and a continuous layerdisposed about said cooling duct.

III k K l

1. An electrical condenser bushing, comprising an axially extendingelectrical conductor, a plurality of concentric, radially adjacentcondenser systems disposed about said electrical conductor, at least oneaxially continuous layer of electrically conductive material disposedbetween at least two radially adjacent condenser systems, which layer isstructurally independent from either adjacent system, and whichfunctions to serially interconnect the adjacent condenser systems, andat least one cooling duct disposed between two adjacent condensersystems, with the at least one axially continuous layer disposedadjacent to one side of the duct, and including another axiallycontinuous layer disposed adjacent to the other side of said duct.
 2. Anelectrical condenser bushing, comprising: an axially extendingelectrical conductor, electrical insulating means disposed about saidconductor, a plurality of radially spaced, axially discontinuous layersof electrically conductive material disposed in said insulating means,about said conductor, a plurality of axially continuous layers ofelectrically conductive material, including a continuous layer disposedabout the outermost axially discontinuous layer, and at least onecontinuous layer disposed intermediate the axially discontinuous layers,to divide the axially discontinuous layers into a plurality of radiallydisposed groups, and at least one cooling duct disposed in theinsulating means, with axially continuous layers disposed immediatelyadjacent to the inner and outer sides of the at least one cooling duct.3. An electrical condenser bushing, comprising: an axially extendingelectrical conductor, electrical insulating means disposed about saidconductor, a plurality of radially spaced, axially discontinuous layersof electrically conductive material disposed in said insulating means,about said conductor, a plurality of axially continuous layers ofelectrically conductive material, including a continuous layer disposedabout the outermost axially discontinuous layer, and at least onecontinuous layer disposed intermediate the axially discontinuous layers,to divide the axially discontinuous layers into a plurality of radiallydisposed groups, a cooling duct disposed adjacent to the conductor, anda continuous layer disposed about said cooling duct.